Document path stability of `f64::abs` in no std crates

[lopopolo]

I tried this code:

#![no_std]

pub fn xxx() {
    f64::abs(1.0);
}

In Rust older than 1.84.

I expected to see this happen: successful compilation since f64::abs is documented to be available since Rust 1.0.

Instead, this happened: compilation failure since f64::abs in no std programs wasn't stabilized until Rust 1.84. This isn't documented anywhere and I had to go issue diving on GitHub.

@RalfJung said the keywords here are documenting path stability vs item stability.

#131304 (comment)

Opening this issue to explore how to document the limited availability of items in no std programs.

In the absence of compiler infrastructure for this, free form text in the prose of the doc comment for the f64 primitive or its abs method would have solved this for me. Maybe in a # Caveats section?

[RalfJung]

Cc @rust-lang/rustdoc @rust-lang/libs-api

[GuillaumeGomez]

This is indeed pretty bad. I'll try to take a look whenever I have time if no one did until then.

[DJMcNab]

Linking to #137578 and #50145, which are both are recent cases where this kind of issue can occur.

I talked about this with @joshtriplett at the RustWeek unconference - in Linebender we have run into similar issues.
Josh also suggested that the other way around would be helpful. That is, somehow displaying in std::f64::abs that it is also available in core. This would be a natural extension of this issue, as this would want to use the same MSRV-awareness infrastructure in that display.
(Ideally, something like Clippy would also suggest that you use are using an item from std which is also in core, which it can only correctly do if it knows when it moved to core).

[lopopolo]

CStr is another such example:

• Rust 1.64.0 - https://doc.rust-lang.org/stable/core/ffi/struct.CStr.html
• Rust 1.0.0 - https://doc.rust-lang.org/stable/std/ffi/struct.CStr.html

And OsStr:

• Rust 1.87.0 - https://doc.rust-lang.org/stable/std/ffi/os_str/struct.OsStr.html
• Rust 1.0.0 - https://doc.rust-lang.org/stable/std/ffi/struct.OsStr.html